The present invention is directed to the detection of dry/wet states of a thermistor bead, in general, and more particularly, to a method and apparatus for detecting a dry/wet state of a thermistor bead utilizing a temperature compensated reference level circuit.
Sensing liquid level in a container, like fuel in an aircraft fuel tank, for example, has been performed using resistive bead type thermistors which have an inverse temperature coefficient. When current is conducted through a thermistor bead in air, i.e. a dry state, the bead increases in temperature and exhibits a low resistance to the current. In contrast, when the thermistor bead is submersed in a liquid, like jet fuel, for example, the bead is cooled and its resistance to current conducted therethrough is increased. Current vs. voltage (I/V) characteristics of a typical thermistor bead at various temperatures is shown in the graph of FIG. 1. It is readily observable from the graph of FIG. 1 that the voltage across the thermistor becomes a viable measurement for detecting a wet vs. dry (wet/dry) state of the thermistor bead as the current conducted therethrough becomes greater than 45 milliamps.
For example, if the bead current is fixed at say 45 ma, and a voltage reference level is set at approximately 3.2 volts, then for all temperatures within the range of xe2x88x9254xc2x0 C. to +74xc2x0 C., a bead voltage greater than the reference level indicates a wet state and vice versa. Thus, by passing a constant current equal to or greater than 45 ma through a thermistor bead and lowering the bead from the top of the container, it may be determined at what level in the container the thermistor bead becomes submersed into the liquid by detecting the wet/dry state thereof based on the voltage across the thermistor bead and a fixed reference voltage.
Known interface circuitry which uses the above described method of detecting the dry/wet state of a thermistor bead is shown in the block diagram schematic of FIG. 2. Referring to FIG. 2, a thermistor bead 10 is coupled between a constant current source 12 and a common or ground return. The constant current source 12 is powered by a power supply 14 and is operative to conduct current through the thermistor bead 10. The voltage across the thermistor bead 10 is sensed by one input of a comparator circuit 16 which is also powered by the supply 14 and common return. A fixed reference voltage is generated by a circuit 18 which is powered by the supply 14. The comparator circuit 16 compares the reference voltage which is coupled to another input thereof with the thermistor bead voltage. When the thermistor bead voltage exceeds the reference voltage, a wet bead state is effected at the output of the comparator 16 and when the thermistor bead voltage is less than the reference voltage, a dry bead state is effected at the output of the comparator 16.
As noted above, to insure proper performance of the thermistor bead and detection circuitry using the above described method, the bead 10 should be biased with a constant current equal to or greater than 45 milliamps where voltage levels across the bead are dry/wet distinct for all practical temperature environments (see the graphs of FIG. 1, for example). For level sensing of combustible liquids in a container with a thermistor bead, the bias current level of 45 milliamps may not be considered safe, and thus unacceptable. For example, the FAA has deemed this bias current level unacceptable in terms of the maximum allowable current that may enter an aircraft fuel tank. Only currents less than 30 milliamps with justification have been deemed acceptable for aircraft fuel tanks.
However, as the bead bias current is lowered to less than 30 ma, it is no longer possible to distinguish a wet bead state from a dry bead state based on the comparison of the bead voltage to a fixed reference voltage across a wide operating temperature range, like xe2x88x9254xc2x0 C. to 74xc2x0 C., for example, especially between a dry state at xe2x88x9254xc2x0 C. and a wet state at 74xc2x0 C. Accordingly, at bead currents less than 30 ma, the thermistor bead may not be an acceptable level measurement sensor for combustible liquids across a wide operating temperature range using interface circuitry implementing the above described traditional method.
In accordance with one aspect of the present invention, a method of detecting a dry/wet state of a thermistor bead comprises the steps of: conducting a bias current through the thermistor bead; measuring a voltage across the thermistor bead in response to the bias current; measuring a temperature in proximity to the thermistor bead; generating a reference voltage in proportion to the measured temperature; and detecting the dry/wet state of the thermistor bead based on the measured and reference voltages.
In accordance with another aspect of the present invention, apparatus for detecting a dry/wet state of a thermistor bead comprises: a first circuit coupled to the thermistor bead for conducting a bias current through the thermistor bead; a temperature sensor for measuring a temperature in proximity to the thermistor bead; a second circuit coupled to the temperature sensor for generating a reference voltage in proportion to the measured temperature of the sensor; and a third circuit coupled to the thermistor bead for measuring a voltage across the thermistor bead in response to the bias current, and also coupled to the second circuit for detecting the dry/wet state of the thermistor bead based on the measured and reference voltages.
In accordance with yet another aspect of the present invention, a system for determining a level of liquid in a container with a thermistor bead comprises: means for disposing the thermistor bead at a height in the container; a temperature sensor disposed in the container for measuring a temperature in proximity to the thermistor bead; a first circuit coupled to the thermistor bead for conducting a bias current through the thermistor bead; a second circuit coupled to the temperature sensor for generating a reference voltage in proportion to the measured temperature of the sensor; a third circuit coupled to the thermistor bead for measuring a voltage across the thermistor bead in response to the bias current, and also coupled to the second circuit for detecting the dry/wet state of the thermistor bead based on the measured and reference voltages and generating a signal indicative thereof; and a fourth circuit for determining the level of liquid in the container based on the bead height and dry/wet state signal.